The continuous evolution of wireless communication leads to networks that cover many different use cases in many different frequency spectra. With a wide array of applications, the targets for reliability and throughput to be fulfilled by the network vary drastically. Moreover, regulatory constraints on the wireless communication, radio frequency planning for the network and the capabilities of devices participating in the network demand appropriate selection and adaptation of the transmission parameters as well as optimization of the modulation-and-coding scheme. In particular, the transmit power control, the adaptive antenna beamforming and the selection of the modulation-and-coding scheme are highly important in order to achieve the target throughput and reliability.
Evolving cellular wireless networks (e.g., fifth generation or 5G networks) overcome the limitations of existing cellular networks by allowing for higher data rates, less latency, less energy consumption and to satisfy the ever-increasing traffic demand. For this purpose, additional spectrum beyond what was previously allocated to existing standards and, in some use cases, denser deployments are applied. The use of higher frequency bands (e.g., including licensed, unlicensed and licensed-shared spectrum) overcomes the scarcity of spectrum resources by enabling wider frequency bandwidths and allows more advanced antenna arrays for massive beamforming. A massive growth in the number of connected devices as well as an increasingly wide range of applications enables a well-functioning networked society, in which information can be accessed and data shared anywhere and anytime, by anyone and anything.
In addition, the evolution of existing technologies (e.g., Wi-Fi or any technology based on IEEE 802.11, as well as the fourth generation, or 4G, cellular wireless networks such as 3GPP Long Term Evolution or LTE) is challenged by the same demands. Multi-antenna technologies have a key role in the design of modern Radio Access Technologies (RATs) due to their well-recognized benefits, e.g. according to IEEE 802.11ac and IEEE 802.11ax. Specifically, they enable array gain, spatial multiplexing and spatial diversity, which lead to improved coverage, capacity, and robustness. The multi-antenna features have significantly contributed to the success of LTE and continue driving its evolution.
Multi-antenna technologies are particularly relevant in high-frequency bands as a counter measure to propagation loss increasing with frequency, e.g., atmospheric attenuation, rain fading, foliage attenuation, reduced penetration of walls, diffraction loss (or fast fading) and obstruction loss (or slow fading). While some of the loss aspects are minor problems for lower frequency bands, their impact becomes severe in centimeter and millimeter wave ranges. For example, the communication range limited by path-loss advantageously reduces the frequency reuse distance. Thus, denser deployment, larger bandwidth and smaller beamwidth (e.g., higher antenna gain) can partially compensate, or even overcompensate, the disadvantage of higher path-loss.
Existing wide area networks primarily use licensed spectrum. The license costs are significant but permit high transmit power, accurate cell planning and full frequency reuse without the need to apply access schemes, such as Listen-Before-Talk (LBT) and/or restricted radio duty cycles, in the licensed spectrum. This ensures good coverage even in areas of sparse deployments. The exclusive use of the spectrum minimizes the risk of delay spikes and maximizes the capacity.
On the other hand, Wi-Fi, LTE License-Assisted Access (LAA), LTE in unlicensed spectrum (LTE-U), MulteFire and other emerging technologies, use unlicensed spectrum. Such technologies permit use cases (e.g., corporate networks, in-house network or inter-vehicle communication) for which licensed spectrum is not applicable or not available, given that any transmitting device ensures fair access to the spectrum, e.g., by means of a coexistence mechanism such as LBT. Energy detection just before a planned transmission burst may reveal that the spectrum is already used by another device. A back-off scheme keeps access delay short while making the spectrum sharing fair. LBT schemes are efficient if occupancy of radio resources is low (e.g., at relatively low transmit power levels and/or a few number of contending stations within range of communication). Correspondingly, for a transmitter in unlicensed spectrum, the maximum allowed transmit power and the power spectrum density is subject to regulatory restrictions. The main purpose for having such restrictions is to establish fair coexistence among the different technologies that are operating in these bands. These restrictions are specific for region and frequency band. Thus, the restrictions highly depend on the specific band allocated to the various applications.
Hence, these factors and restrictions pose several design challenges on evolving RATs but also provide opportunities for further use cases.